Forces on Aircraft
Lift is the upward force created by the effect of airflow as it passes over and under the wings. Aircraft fly as a result of and in spite of many forces acting on the aircraft. A force is a push or pull by something on something. When standing we push down on the floor and the floor pushes up on our feet. Forces always act in pairs, on different bodies, and in opposite directions.
There are four major forces acting on an aircraft in flight. Lift is the upward force allowing the aircraft to fly. Lift counters the weight that is holding the aircraft down. Thrust is the push or pull forward that allows the aircraft to move. Drag is a slowing force. Drag always acts in a direction opposite thrust.
When air flows over the curved upper surface of a wing, it increases in velocity. This increase reduces the pressure above the wing and produces the upward force of lift.
Lift is the key aerodynamic force. During flight, pressures on the upper and lower surfaces of the wing are not the same. Although several factors contribute to this difference, the shape of the wing is the principal one. Several factors influencing lift can be controlled by the aircraft's wing design.
You can control lift by changing the angle of the airplane's wing relative to the wind or by increasing or decreasing airspeed. You can also change the shape of the wing by lowering the flaps which extend the area of the wing. Keep in mind that anytime you do something to change lift, drag is affected. For a given lift-to-drag ratio, if you increase lift, drag increases. Drag is always a by-product of lift.